1. Field of the Invention
The present invention relates generally to a method of driving an interline transfer type charge-coupled device (CCD) imager. More particularly, the present invention relates to a method of driving an interline transfer type CCD imager which operates in a field accumulation mode.
2. Description of the Prior Art
In an interline transfer type CCD imager, signals are generally read out by a 2/1-interlaced reading system in which two fields constitute one frame. The accumulation of signals in each of plural light receiving pixels is conducted in either a frame accumulation mode in which one light accumulation period for each pixel coincides with one frame; or a field accumulation mode in which one light accumulation period coincides with one field. The field accumulation mode has a high dynamic resolution, as is described in "Proc . of Conf. on Charge Coupled Device Tech. and Appl., p. 152 (1976)", and is commonly used in movie cameras.
Methods of driving a CCD imager in the field accumulation mode are generally classified as being a single transfer (ST) method or a double transfer (DT) method (See, lot example, Technical Report of Television Engineering Society of Japan, TEBS87-3 (1983)). In the ST method, the process of reading signals from the light receiving pixels into a vertical transfer unit is conducted in one step after the light accumulation period of one field . In contrast, in the DT method, the process of reading signals from the light receiving pixels is conducted in two steps.
In the ST method, as is graphically represented in FIGS. 5A and 5B, the respective signal charges S.sub.1 and S.sub.2 from all of the light receiving pixels PD are transferred to a vertical transfer unit V (FIG. 5A). Thereafter, the signal charges S.sub.1 and S.sub.2 of two vertically adjacent pixels are added in the vertical transfer unit V (FIG. 5B see also, for example, Japanese patent publication No. 62-40910). The addition in odd fields is conducted in a manner different from that in even fields, as is known, thereby complying with the interlaced reading system.
In the DT method, as is graphically represented in FIGS. 6A and 6B, the signal charges S.sub.1 of half of the light receiving pixels are transferred initially to the vertical transfer unit V (FIG. 6A). Typically, this half of the light receiving pixels consists of every other pixel along the vertical direction as shown in FIG. 6A. Thereafter, the signal charges S.sub.1 are transferred forward or backward in the vertical transfer unit V by a distance equal to one pixel pitch. The signal charges S.sub.2 from the remaining light receiving pixels are subsequently read out and are added to the previously transferred signal charges S.sub.1 (FIG. 6B). The transfer direction in odd fields along which signal charges S.sub.1 are transferred by one pixel pitch is made different from that in even fields, thereby complying with the interlaced reading system.
One particular drawback associated with the above-described ST method is that signal charges of all of the light receiving pixels must first be retained in the vertical transfer unit V without mixing the signal charges. This often causes a decrease in the maximum transferable amount of signal charges. Problems can arise, for example, when a vertical transfer unit V is operated by a four-phase driving system. The signal charges transferred from one pixel must be held under one electrode in the vertical transfer unit V and, therefore, the signal charges are strongly affected by a fringe electric field as compared to another method in which the signal charges transferred from one pixel are held under two or more electrodes. Hence, the amount of signal charges transferred from each pixel in the ST method typically will be less than half of that in another method. As a result, even after signal charges transferred from two pixels are added according to the ST method, the amount of the resulting signal charges will not reach the amount of signal charges as compared to another method. As the difference in the available amounts of signal charges to be added in the ST method becomes greater, the degree of the reduction in signal charges as compared to other methods becomes even larger as will be appreciated.
Using the DT method, typically the maximum transferable amount of signal charges is not a problem, but signal charges in the vertical transfer unit V must be transferred in reverse directions depending on whether it is in relation to an odd or even field. Such reverse charge transfer of signal charges often causes a problem of erroneously transferring signal charges. Such problem does not exist in a forward charge transfer. For example, in a situation where there is a small potential dip in one portion of the vertical transfer unit V, forward charge transfers cause this potential dip to be filled always with a signal of the former stage, so as to prevent the potential dip from appearing externally. If, however, a reverse charge transfer is first conducted, the signal charge in the potential dip is swept away. This swept amount of signal charge from the potential dip is recovered by a signal of the next stage, resulting in an erroneous charge transfer which adversely affects the quality of output signals.
The assignee of this application has developed an imager which can eliminate the above-mentioned problems of the ST and DT methods (Japanese patent application No. 1-298973). In such an improved imager (represented in part by FIGS. 7A and 7B), the potential distribution in a vertical transfer unit V is controlled such that the signal charges of two vertically adjacent pixels will be added. Then, signal charges of all light receiving pixels are read out sequentially or simultaneously to be added. As a result, the above-described problems are eliminated, i.e., the problems of a reduced maximum transferable amount of signal charges, and of erroneous charge transfer caused by a reverse charge transfer. However, when signal charges of the light receiving pixels PD are read out, most of the electrodes of the vertical transfer unit V (in the four-phase driving system, for example, three electrodes) are set to a high potential. Hence, the potentials of light receiving pixels become unstable. Therefore, the improved imager has a drawback that, when reading out signal charges of light receiving pixels, the charge transfer may erroneously function.
An object of the present invention is to provide a method of driving an interline transfer type CCD imager by which the maximum transferable amount of signal charges in the field accumulation mode can be increased, and the above-described problems of an erroneous charge transfer due to a reverse charge transfer or the reading of signal charges from light receiving pixels can be eliminated.